In general, beam combining of multiple lasers allows overcoming power and energy limitations of each individual laser. Beam combining of multiple laser signals can be currently achieved using a variety of methods such as active coherent phasing, incoherent spectral combining, passive self-locked combining, or incoherent spatial addition of multiple laser beams. In each of these methods, each parallel laser channel generates an identical signal and all parallel output signals are then combined with the total combined power proportional to the sum of individual powers from all N laser channels. The maximum achievable power can never exceed this total sum, as fundamentally limited by the power conservation law. These beam combining methods can be applied to continuous wave signals as well as pulsed signals. When these currently used methods are used to combine pulsed beams, the combined energy per each pulse can never exceed the sum of individual pulse energies from all the channels.
In case of pulsed signals, it is possible to achieve simultaneous pulse combining and time-domain energy redistribution such that in a combined beam pulse repetition rate is reduced and, therefore, combined energy per pulse now increases proportionally to both this repetition rate decrease and to the total sum of individual pulses from all the channels. This increases combined pulse energy faster than a linear proportionality to a number of parallel channels N. As described in applicant's previous work, this increase could be proportional to N2 of the number of channels, thus significantly reducing combined laser array size and complexity for reaching the same combined pulse energy as with the above-described methods. Further details regarding this previous work can be found, for example in U.S. patent application Ser. No. 14/403,038 which is incorporated in its entirety herein. The technique described in this previous work is applicable only to periodically pulsed signals. Redistribution of energy between the pulses in time domain requires that a beam-combining element would provide a time delay longer than the time duration over which pulse energy is redistributed. This time delay is always associated with the size of the combining element. For example, in the N2 combining method described in applicant's previous work, a Fabry-Perot or any other configuration of a resonant cavity can be used, with its round-trip length L=c·ΔT, where ΔT is the each-channel periodic-signal repetition period, as shown in FIG. 1. This means that pulse repetition rate at the input of the system has to be high, in order to be compatible with a practically short length of the combining cavity. This means that achieving high energy at high repetition rates is associated with impractically high average powers. For example, a 3 m long roundtrip cavity corresponds to 100 MHz repetition rate of the periodic pulse input. Assuming that each pulse in the input periodic signal is at about 1 mJ of energy (expected energy of ˜1 ns pulse at the maximum of each parallel amplification channel), then the average power per each amplified beam from each parallel channel is approximately 100 kW.
This section provides background information related to the present disclosure which is not necessarily prior art.